As overall stability is one of the key indices affecting the hoisting performance of a hoisting machine, when a hoisting machine is designed, a counterweight mass tends to be increased to a bearable limit of an axle so as to improve the overall stability to a maximum extent. For a large-tonnage product, a combined counterweight manner tends to be used to improve the hoisting performance of a hoisting machine.
In principle, what actually affects the stability of a crane is a moment produced by a counterweight relative to a center of rotation. When a counterweight is set to be Q, and a distance of a center of gravity of the counterweight relative to a center of rotation is L, a moment of the counterweight acting on a center of rotation is: w=Q×L. Thus, the overall stability of a crane may also be improved by increasing a distance between a center of gravity of a counterweight and a center of rotation.
For a small-tonnage product limited to an axle load, a counterweight mass significantly constrains the overall stability. Thus, to increase a distance between a center of gravity of a counterweight and a center of rotation is of vital importance for ensuring the overall hoisting performance. However, if various aspects such as distribution of an axle load, stability of a vehicle trip, safety and appearance molding are taken into account, the retraction distance of the counterweight is extremely limited. Accordingly, a solution widely adopted in industry is to retract a center of gravity of a counterweight during operation, and retrieve the same in a travelling state.
At present, there are two methods of effectuating promoting the overall stability by a counterweight action as follows:
1) Combined counterweight: limited by an axle load of a hoisting machine and considering the safety, a hoisting machine is only with less amount of counterweight, or even with no counterweight in the process of travel or transition. After a vehicle is positioned at a hoisting work area, a counterweight is hitched on a hoisting machine by means of a hydraulic cylinder or in other manners, and the overall hoisting stability is improved in such a manner as to increase the weight of a counterweight. However, the technical disadvantages of a combined counterweight solution comprise the following two aspects: a. the counterweight needs an additional vehicle for transportation so that the hoisting cost is increased; b. movement and hitching of the counterweight are realized in a hoisting manner, which also increases the hoisting cost.
2) Mobile counterweight: with certain weight for a counterweight of a hoisting machine, the overall stability is improved by an external force moving the counterweight outwards by certain distance relative to a center of rotation. There are two technical means in the realization manner, respectively a guide rail type and a connection rod type.
FIG. 1 illustrates a guide rail type counterweight movement solution, with a solid line indicating a travel (or transition state), and a dotted line indicating an operation state. Two guide rails A14 are respectively fixed to both sides at the bottom of a winch box A12 of a hoisting machine, and a slide block A13 is fixed to a counterweight A11. When the slide block A13 moves along the guide rail A14 under the impulse of an cylinder, the counterweight also moves by the same distance D.
FIG. 2 illustrates a connection rod type counterweight movement solution, with a solid line indicating a travel (or transition state), and a dotted line indicating an operation state. Two connection rods A25 are arranged in parallel, with an upper articulation point A23 fixed to the winch box and a lower articulation point A24 fixed to a counterweight A21, so as to effectuate an overall retraction distance D of the counterweight A21 by the cylinder pushing the lower articulation point A24 of the connection rods.
However, the aforementioned two technical means of a mobile counterweight are both present with the defect of a short movement distance (a movement distance of a guide rail type counterweight is restricted by rigidity of the guide rail, while the connection rod type solution is restricted by the stability of an action in the movement process). For a small-tonnage product with a light counterweight itself, if the movement distance is short, it is not prominent for the effect of promoting the overall stability, and it is impossible to possess an actual value in use with respect to the cost increased by a mobile counterweight.